Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium, glass, fabric, metal, or other objects as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a visible image. Note that the visible image may not be visible to the naked eye depending on the composition of the toner particles (e.g. clear toner).
After the latent image is developed into a visible image on the photoreceptor, a suitable receiver is brought into juxtaposition with the visible image. A suitable electric field is applied to transfer the toner particles of the visible image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (“fuse”) the print image to the receiver. Plural print images, e.g. of separations of different colors, are overlaid on one receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic (EP) printers typically transport the receiver past the photoreceptor to form the print image. The direction of travel of the receiver is referred to as the slow-scan, process, or in-track direction. This is typically the vertical (Y) direction of a portrait-oriented receiver. The direction perpendicular to the slow-scan direction is referred to as the fast-scan, cross-process, or cross-track direction, and is typically the horizontal (X) direction of a portrait-oriented receiver. “Scan” does not imply that any components are moving or scanning across the receiver; the terminology is conventional in the art.
Electrophotographic developer can include toner particles and magnetic carrier particles, and is transported past the photoreceptor by a development member. Developer is compressible, and the image quality of the print image is strongly correlated with developer density. However, existing methods for measuring developer density and other properties require off-line processing, so it cannot provide the data necessary to maintain image quality on-line and thereby improve throughput of a printer.
Commonly-assigned U.S. Publication No. 2002/0168200 ('200) by Stelter et al., the disclosure of which is incorporated herein by reference, describes determining developer mass velocity by, among other things, measuring developer flow rate and developer mass area density (DMAD). Measuring flow rate requires collecting developer in a hopper from a bench-top toning station, and measuring DMAD requires abruptly stopping the toning station. Although these operations are useful, neither is suitable for an operating machine; both are invasive procedures that require the machine to be partially disassembled.
U.S. Pat. No. 6,498,908 to Phillips et al. describes a charge measurement device for measuring charge transfer between a high-voltage power supply and a developing device during an imaging operation. However, charge transfer can occur for various reasons, and it can be difficult to determine which reason affects a particular charge transfer. A single measurement is therefore not always enough information to fix a problem.
U.S. Pat. No. 4,519,696 to Bruyndonckx et al. describes inductive measurement of toner concentration in a developer mixer. However, toner concentration and developer flow rate both affect the percentage of carrier particles in the measurement volume of a sensor, and therefore the toner concentration measured by that sensor.
Commonly-assigned U.S. Pat. No. 4,987,453, the disclosure of which is incorporated herein by reference, describes various embodiments of a capacitive sensor using decay time under applied bias as a signal. Commonly-assigned U.S. Pat. Nos. 5,532,802, 5,463,449, 5,285,243, 5,235,388, 5,122,842, and 5,006,897, and “A Piezoelectric sensor for in situ monitoring of xerographic developers” (by Rimai et al.; Journal of Imaging Science and Technology vol. 39, 1995, pp 136-141), the disclosures of all of which are incorporated herein by reference, describe various embodiments of piezoelectric capacitive sensors. For example, '897 and '842 describe sensors for measuring the toner mass-deposition rate and charge-to-mass ratio. Although useful, these sensors do not measure other significant characteristics of the developer, such as flow rate and toner concentration, and do not separate out different effects from each other.
There is a need, therefore, for an improved way of measuring toner concentration in an electrophotographic system, separating out different effects from each other.